14
SYSTEM
3.4 VAPOR PRESSURE SENSOR
The vapor pressure sensor (
) on the ATMOS 41 is located behind the circular Teflon
™
membrane in the same housing as the sonic transducers. The Teflon protects the sensor
from liquid water and dust while allowing water vapor to freely pass to the sensor and
equilibrate with air vapor pressure. The sensor measures relative humidity and temperature
and computes vapor pressure as the saturation vapor pressure at sensor temperature
multiplied by the relative humidity.
Figure 5 Vapor Pressure Sensor
If the relative humidity of the air is desired, it can be computed using
.
Equation 1
(
)
RH
e T
e
,
r air
s
air
a
=
where
e
a
is the vapor pressure of the air, from the ATMOS 41, and
e
s
(T
air
)
is saturation vapor
pressure at the air temperature given by the ATMOS 41.
The saturation vapor pressure is calculated using the Magnus-Tetens equation (
with the following coefficients described by Buck (1981).
Equation 2
exp
e T
a
c
T
bT
s air
air
air
=
+
e
o
Water
a
= 0.611 kPa
b
= 17.502
c =
240.97 °C
T
air
= NA
Ice
a
= NA
b
= 21.87
c =
265.5 °C
T
air
= Temperature in °C
Vapor pressure sensor
Teflon screen
